Among patients who are the subjects of blood coagulation tests, there are patients who are treated with heparin as a therapeutic drug for thrombi. Meanwhile, there are patients for whom a catheter connected to a heparin lock which is filled with saline containing heparin is used, in order to prevent blood coagulation reactions from occurring in the catheter when drip infusion is performed. Test plasma collected from these patients may contain extrinsic heparin. Use of such test plasma for measuring an activated partial thromboplastin time (APTT) results in a prolonged blood coagulation time due to the influence of the heparin.
U.S. Pat. No. 5,262,325 discloses a method for removing the influence of extrinsic heparin contained in test plasma on an APTT measurement. In the method, the test plasma is neutralized by a heparin neutralizer and then an APTT is measured, thereby removing the influence of heparin contained in the test plasma. However, according to the method disclosed in U.S. Pat. No. 5,262,325, it is necessary to add in advance a heparin neutralizer in the test plasma. Therefore, the steps of the APTT measurement have been burdensome. Moreover, according to the method disclosed in U.S. Pat. No. 5,262,325, it is necessary to always consider possible presence of heparin in the test plasma, when performing an APTT measurement. In particular, in medical sites, when performing an emergency APTT measurement, an APTT measurement may be performed without checking the presence or absence in the test plasma of extrinsic heparin.
Moreover, in recent years, also in a cross mixing test which uses an APTT measuring reagent, the influence of extrinsic heparin contained in test plasma has been a problem (Kensa to gijutu, vol. 34, no. 8, August, 2006, pp. 738-739). A cross mixing test is a test that detects coagulation factor deficiency in a patient and the presence of a blood coagulation inhibitor in test plasma, by measuring the blood coagulation time of mixed plasma obtained by mixing the test plasma and normal plasma. As described above, when extrinsic heparin is contained in test plasma, the blood coagulation time is prolonged. Therefore, also in a cross mixing test, if extrinsic heparin is contained in the test plasma, the presence of a blood coagulation inhibitor that is an intrinsic factor may not be detected accurately.